1. Field of the Invention
The invention relates generally to a process and apparatus for servicing nuclear reactors. More particularly, the invention relates to the servicing of jet pump hold down beams which are used to secure the position of reactor coolant jet pumps in the reactor.
2. Description of the Related Art
Jet pumps are used in a variety of different types of nuclear reactors for circulating moderator fluid such as cooling water through a reactor core located within the pressure vessel of the reactor structure. Reactors such as boiling water reactors can employ as many as twenty or more downwardly-directed jet pumps mounted in an annulus between the reactor vessel wall and the reactor shroud. Each of the pumps recirculates reactor core moderator coolant fluid such as water by forcing high volumes of the fluid through a venturi nozzle, which then draws surrounding fluid into the pump for circulation. Fluid volumes of on the order of about 94,500 l./min. (25,000 gal./min.) can be obtained from each of the jet pumps. Because of the forces associated with the circulation of such high volumes of fluids, jet pump hold down beams are provided to secure the pump in position throughout pump operation. The hold down beams are pre-tensioned with up to about a 13,600 kg. (30,000 lb.) preload per beam under shoulders formed in a pair of opposed jet pump riser brackets.
Jet pump hold down beams of existing boiling water reactors typically consist of three components: A beam body, a pair of opposed tabs extending outwardly from the beam body, and a threaded beam bolt extending through a correspondingly-threaded channel formed in the beam body. Beam tensioning and detensioning is accomplished by rotation of the beam bolt in the appropriate direction. A beam bolt retainer assembly can be provided to stabilize the position of the beam and accompanying beam bolt with respect to the jet pump. The retainer assembly includes a retainer plate having a channel formed therein for receiving the beam bolt and abutting against a flanged end of the beam bolt, and a retainer bolt for coupling the retainer plate to the jet pump. A keeper collar can be provided to rotatably fix the position of the beam bolt within the threaded channel of the beam. The keeper collar is insertable over the upper end of the beam bolt and tack welded to a weld plate coupled to the beam to provide a force resistive to rotational movement of the beam bolt.
Several problems with existing jet pump hold down beams in boiling water reactor plants have been discovered during the course of routine reactor servicing, such as during refueling outages. One problem has been the occurrence of cracking of the tack welds of the beam bolt keepers. Tack weld cracking necessitates re-welding of the keeper to the beam, a difficult procedure to perform under water and in the narrow confines of the reactor annulus. A more serious problem has been the discovery of hold down beam cracks and fissures. Metallurgical studies have revealed the cause of the beam failure to be the result of intergranular stress corrosion cracking, arising from chemical interaction between the beam alloy material and the moderator fluid. These cracked beams must be removed from the jet pump and replaced with new beams. In order to minimize reactor down time, it is desirable to conduct hold down beam replacement during reactor fuel changeovers. However, because the hold down beams are positioned in a relatively narrow annulus between the core shroud and the reactor vessel wall and are submerged in radioactively contaminated moderator fluid at a depth of about 10 meters (30 feet) below the level of the reactor vessel flange, jet pump hold down beam replacement has heretofore been a labor-intensive, time consuming, expensive, and hazardous procedure.
Many of the difficulties associated with jet pump hold down beam replacement arise from the considerable health and safety considerations associated with the design and construction of power generating nuclear reactors. For example, the reactor vessel of a conventional boiling water reactor is submerged in the concrete floor of a floodable refueling cavity and is covered by a vessel head detachably mounted to the vessel flange. In order to obtain access to the jet pump hold down beams of the boiling water reactor, the refueling cavity must first be flooded with moderator fluid such as water so that the vessel head is immersed in about 15-20 meters of moderator fluid. Only after the refueling cavity has been flooded can the vessel head be safely uncoupled from the vessel flange and various reactor components removed therefrom. Removal of the vessel head and reactor components is accomplished from a refueling platform extending across the top of the refueling cavity. Thereafter, in known servicing techniques, the fluid is drained from the refueling cavity and the cavity walls and floor are scrubbed and decontaminated. A service platform from which hold down beam servicing is to be effected is then assembled across the reactor flange, and designated hold down beams are then removed and replaced. A bolt keeper is tack welded to the top of the beam, and proper moderator fluid chemistry and clarity is restored. The service platform is then removed, and the vessel head is re-installed on the vessel flange. Due to the difficulties presented by this known servicing procedure, the time required to replace the hold down beam of a single jet pump has in the past been on the order of about three days. As there are twenty or more jet pumps spaced about the periphery of the reactor annulus, and because prudence dictates the replacement of all of the hold down beams where abnormalities are found with one or more of the beams, it is readily apparent that jet pump hold down beam servicing is a significant service procedure, resulting in a substantial amount of reactor down time.
Various tools have been used in the past for effecting hold down beam replacement. For example, U.S. Pat. No. 4,406,047 discloses an apparatus and procedure for replacing the jet pump hold down beam in which the beam is forced out from the retainer channel following beam rotation and beam bolt loosening by a separate apparatus. A principal drawback to this arrangement is that the beam retainer assembly remains bolted in place to the upper end of the jet pump. Therefore, in order to install a replacement hold down beam, the lower end of the beam bolt of the replacement beam must first be inserted into the retainer plate in order to properly position the beam with respect to the upper end of the jet pump and the opposed bracket members extending therefrom. Because of the narrow dimensions of the jet pump annulus, manipulation of the replacement beam so as to properly fit within the retainer assembly is difficult to accomplish.
In an effort to expedite hold down beam replacement, apparatus for cutting the beam retainer plate from the jet pump have been proposed. One example of such an apparatus is disclosed in U.S. Pat. No. 4,499,691. This patent discloses a device for cutting the hold down beam retainer plate so that a portion of the severed plate is removed with the beam, whereas the remaining plate-anchoring portion is left coupled to the jet pump. However, because this apparatus involves cutting of the retainer plate adjacent its position of installation on top of the jet pump, operation of this apparatus is accompanied by the risk that metal shards produced incident to retainer plate cutting will become entrained in the surrounding reactor moderator fluid and pumped throughout the reactor, thereby increasing the risk of reactor failure and attendant down time.
In view of the foregoing limitations and drawbacks in the prior art, an object of the present invention is to provide a more expedient and cost-effective apparatus and process for the removal and replacement of jet pump hold down beams.
Another object of the invention is to provide a new method and apparatus for effecting jet pump hold down beam removal and replacement which subjects service personnel to considerably reduced levels of radiation than has heretofore been possible during hold down beam replacement procedures.
Yet another object of the present invention is to provide a variety of end effector apparatus for performing the removal and replacement of jet pump hold down beams from a position remote from the jet pumps, in which the end effectors define a family of jet pump servicing apparatus which are interchangeably connected to the lower portion of a readily manipulable positioning device.
Yet still another object of present invention is to provide a method and apparatus for rotatably fixing the position of the beam bolt of a replacement hold down beam following installation of the replacement beam that avoids altogether the use of welding at the site of beam installation.